Hypersensitive response elicitor from Erwinia chrysanthemi

ABSTRACT

The present invention relates to an isolated protein or polypeptide corresponding to a protein or polypeptide in Erwinia chrysanthemi which elicits a hypersensitive response in plants. The encoding DNA molecule alone in isolated form or either in an expression system, a host cell, or a transgenic plant are also disclosed. Another aspect of the present invention relates to a method of imparting pathogen resistance to plants by transforming a plant with the DNA molecule of the present invention.

This work was supported by NRI Competitive Grants Program/USDA grants 91-37303-6321 and 94-37303-0734.

This application is a division of U.S. patent application Ser. No. 08/484,358, filed Jun. 7, 1995 now U.S. Pat. No. 5,850,015.

FIELD OF THE INVENTION

The present invention relates to the Hypersensitive Response Elicitor from Erwinia chrysanthemi and its uses.

BACKGROUND OF THE INVENTION

Interactions between bacterial pathogens and their plant hosts generally fall into two categories: (1) compatible (pathogen-host), leading to intercellular bacterial growth, symptom development, and disease development in the host plant; and (2) incompatible (pathogen-nonhost), resulting in the hypersensitive response, a particular type of incompatible interaction occurring, without progressive disease symptoms. During compatible interactions on host plants, bacterial populations increase dramatically and progressive symptoms occur. During incompatible interactions, bacterial populations do not increase, and progressive symptoms do not occur.

The hypersensitive response is a rapid, localized necrosis that is associated with the active defense of plants against many pathogens (Kiraly, Z., "Defenses Triggered by the Invader: Hypersensitivity," pages 201-224 in: Plant Disease: An Advanced Treatise, Vol. 5, J. G. Horsfall and E. B. Cowling, ed. Academic Press New York (1980); Klement, Z., "Hypersensitivity," pages 149-177 in: Phytopathogenic Prokaryotes, Vol. 2, M. S. Mount and G. H. Lacy, ed. Academic Press, New York (1982)), which are hereby incorporated by reference). The hypersensitive response elicited by bacteria is readily observed as a tissue collapse if high concentrations (≧107 cells/ml) of a limited host-range pathogen like Pseudomonas syringae or Erwinia amylovora are infiltrated into the leaves of nonhost plants (necrosis occurs only in isolated plant cells at lower levels of inoculum) (Klement, Z., "Rapid Detection of Pathogenicity of Phytopathogenic Pseudomonads," Nature 199:299-300; Klement, et al., "Hypersensitive Reaction Induced by Phytopathogenic Bacteria in the Tobacco Leaf," Phytopathology 54:474-477 (1963); Turner, et al., "The Quantitative Relation Between Plant and Bacterial Cells Involved in the Hypersensitive Reaction," Phytopathology 64:885-890 (1974); Klement, Z., "Hypersensitivity," pages 149-177 in Phytopathogenic Prokaryotes, Vol. 2., M. S. Mount and G. H. Lacy, ed. Academic Press, New York (1982), which are hereby incorporated by reference). The capacities to elicit the hypersensitive response in a nonhost and be pathogenic in a host appear linked. As noted by Klement, Z., "Hypersensitivity," pages 149-177 in Phytopathogenic Prokaryotes, Vol. 2., M. S. Mount and G. H. Lacy, ed. Academic Press, New York which is incorporated by reference, these pathogens also cause physiologically similar, albeit delayed, necroses in their interactions with compatible hosts. Furthermore, the ability to produce the hypersensitive response or pathogenesis is dependent on a common set of genes, denoted hrp (Lindgren, P. B., et al., "Gene Cluster of Pseudomonas syringae pv. `phaseolicola` Controls Pathogenicity of Bean Plants and Hypersensitivity on Nonhost Plants," J. Bacteriol. 168:512-22 (1986); Willis, D. K., et al., "hrp Genes of Phytopathogenic Bacteria," Mol. Plant-Microbe Interact. 4:132-138 (1991), which are hereby incorporated by reference). Consequently, the hypersensitive response may hold clues to both the nature of plant defense and the basis for bacterial pathogenicity.

The hrp genes are widespread in gram-negative plant pathogens, where they are clustered, conserved, and in some cases interchangeable (Willis, D. K., et al., "hrp Genes of Phytopathogenic Bateria," Mol. Plant-Microbe Interact. 4:132-138 (1991); Bonas, U., "hrp Genes of Phytopathogenic Bacteria," pages 79-98 in: Current Topics in Microbiology and Immunology: Bacterial Pathogenesis of Plants and Animals--Molecular and Cellular Mechanisms, J. L. Dangl, ed. Springer-Verlag, Berlin (1994), which are hereby incorporated by reference). Several hrp genes encode components of a protein secretion pathway similar to one used by Yersinia, Shigella, and Salmonella spp. to secrete proteins essential in animal diseases (Van Gijsegem, et al., "Evolutionary Conservation of Pathogenicity Determinants Among Plant and Animal Pathogenic Bacteria," Trends Microbiol. 1:175-180 (1993), which is incorporated by reference). In E. amylovora, P. syringae, and P. solanacearum, hrp genes have been shown to control the production and secretion of glycine-rich, protein elicitors of the hypersensitive response (He, S. Y., et al. "Pseudomonas Syringae pv. Syringae Harpin_(Pss) : a Protein that is Secreted via the Hrp Pathway and Elicits the Hypersensitive Response in Plants," Cell 73:1255-1266 (1993), Wei, Z.-H., et al., "HrpI of Erwinia amylovora Functions in Secretion of Harpin and is a Member of a New Protein Family," J. Bacteriol. 175:7958-7967 (1993); Arlat, M. et al. "PopA1, a Protein Which Induces a Hypersensitive-like Response on Specific Petunia Genotypes, is Secreted via the Hrp Pathway of Pseudomonas solanacearum," EMBO J. 13:543-553 (1994), which are hereby incorporated by reference).

The first of these proteins was discovered in E. amylovora Ea321, a bacterium that causes fire blight of rosaceous plants, and was designated harpin (Wei, Z.-M., et al, "Harpin, Elicitor of the Hypersensitive Response Produced by the Plant Pathogen Erwinia amylovora," Science 257:85-88 (1992), which is incorporated by reference). Mutations in the encoding hzpN gene revealed that harpin is required for E. amylovora to elicit a hypersensitive response in nonhost tobacco leaves and incite disease symptoms in highly susceptible pear fruit. The P. solanacearum GMI1000 PopA1 protein has similar physical properties and also elicits the hypersensitive response in leaves of tobacco, which is not a host of that strain (Arlat, et al. "PopA1, a Protein Which Induces a Hypersensitive-like Response on Specific Petunia Genotypes, is Secreted via the Hrp Pathway of Pseudomonas solanacearum,"EMBO J. 13:543-553 (1994), which is incorporated by reference). However, P. solanacearum popA mutants still elicit the hypersensitive response in tobacco and incite disease in tomato. Thus, the role of these glycine-rich hypersensitive response elicitors can vary widely among gram-negative plant pathogens.

E. chrysanthemi is unlike the bacterial pathogens that typically elicit the hypersensitive response, because it has a wide host range, rapidly kills and macerates host tissues, and secretes several isozymes of the macerating enzyme pectate lyase (Barras, F., et al., "Extracellular Enzymes and Pathogenesis of Soft-rot Erwinia," Annu. Rev. Phytopathol. 32:201-234 (1994), which is incorporated by reference). Nevertheless, pectic enzyme secretion pathway mutants of E. chrysanthemi EC16 cause a typical hypersensitive response (Bauer, D. W., et al., "Erwinia chrysanthemi hrp Genes and Their Involvement in Soft Rot Pathogenesis and Elicitation of the Hypersensitive Response," Mol. Plant-Microbe Interact. 7:573-581 (1994), which is incorporated by reference). Furthermore, elicitation of the hypersensitive response by E. chrysanthemi is dependent on a hrp gene that is conserved in E. amylovora and P. syringae and functions in the secretion of the E. amylovora harpin (Wei, Z.-H., et al., "HrpI of Erwinia amylovora Functions in Secretion of Harpin and is a Member of a New Protein Family," J. Bacteriol. 175:7958-7967 (1993)i; Bauer, D. W., et al., "Erwinia chrysanthemi hap Genes and Their Involvement in Soft Rot Pathogenesis and Elicitation of the Hypersensitive Response," Mol. Plant-Microbe Interact. 7:573-581 (1993), which are hereby incorporated by reference).

SUMMARY OF THE INVENTION

The present invention relates to an isolated protein or polypeptide corresponding to a protein or polypeptide in Erwinia chrysanthemi which elicits a hypersensitive response in plants. The encoding DNA molecule in isolated form or in either an expression system, a host cell, or a transgenic plant is also disclosed.

Another aspect of the present invention relates to a method of imparting pathogen resistance to plants by transforming them with a DNA molecule encoding the protein or polypeptide capable of eliciting a hypersensitive response in plants and corresponding to a protein or polypeptide in Erwinia chrysanthemi which elicits hypersensitive response in plants.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the DNA sequence of hrpN_(Ech) and predicted amino acid sequence of its product. Underlined are the putative ribosome binding site, the N-terminal amino acids confirmed by sequencing the product of pCPP2172, and a potential rho-independent transcription terminator. The location and orientation of two Tn5-gusA1 insertions are also indicated and are numbered according to their location in the hrpN_(Ech) open reading frame. The accession number for hrpn is L39897.

FIG. 2 compares the predicted amino acid sequences of HrpN_(Ech) and HrpN_(Ea). The predicted amino acid sequences of the Erwinia chrysanthemi ("Ech") and E. amylovora ("Ea") hrpN products were aligned by the Gap Program of the Genetics Computer Group Sequence Analysis Software Package (Devereaux, J., et al., "A Comprehensive Set of Sequence Analysis Programs for the VAX," Gene 12:387-395 (1984)). Two dots denote higher similarity than one dot.

FIG. 3 is an SDS polyacrylamide gel of purified HrpN_(Ech). Purified HrpN_(Ech) was solubilized in SDS loading buffer, electrophoresed through a 12% polyacrylamide gel, and stained with Coomassie Brilliant Blue. Lane 1, molecular weight markers (mid range markers from Diversified Biotech, Boston, Mass.) with size in kD shown to the left; lane 2, HrpN_(Ech).

FIG. 4 shows the response of tobacco leaf tissue to purified HrpN_(Ech). Leaf panel 1 was infiltrated with a suspension of purified HrpN_(Ech) at a concentration of 336 μg/ml of 5 mM MES (morpholinoethanesulfonic acid), pH 6.5. Panel 2 was infiltrated with buffer alone. The tissue in panel 1 collapsed 18 hrs. later. The leaf was photographed 24 hrs. after infiltration using a cross-polarized transilluminator, which enhances black and white visualization by making necrotic, desiccated areas that are typical of the hypersensitive response appear black.

FIG. 5 shows a tobacco leaf showing that Erwinia chrysanthemi hrpN mutants do not elicit the hypersensitive response unless complemented with hrpN+pCPP2174. Bacteria were suspended at a concentration of 5×10⁸ cells/ml in 5 mM MES, pH 6.5, and infiltrated into a tobacco leaf. The leaf was photographed 24 hr later using cross-polarized transillumination as in FIG. 4. Panels and strains: 1, E. chrysanthemi CUCPB5006 ΔpelABCE); 2, E. chrysanthemi CUCPB5045 (ΔpelABCE hrpN_(Ech) 546::Tn5-gusA1); 3, E. chrysanthemi CUCPB5045(pCPP2174); 4, buffer alone; 5, E. chrysanthemi CUCPB5046 (ΔpelABCE hrpN_(Ech) 439:Tn5-gusA1); 6, E. chrysanthemi CUCPB5046(pCPP2174).

FIGS. 6A-D shows Saintpaulia leaves with rapid necrosis elicited by HrpN_(Ech) and HrpN_(Ech) ⁺ Pel-deficient E. chrysanthemi strains. Leaves were inoculated with bacteria at a concentration of 3×10⁸ per milliliter in 5 mM MES, pH 6.5, or purified HrpN_(Ech) at a concentration of 336 μg/ml and photographed 24 hr later as in FIG. 4. Buffer controls elicited no visible response (not shown) Leaves and treatments: FIG. 6A, E. chrysanthemi CUCPB5006 ΔpelABCE); FIG. 6B, E. chrysanthemi CUCPB5045 (ΔpelABCE hrpN_(Ech) 546:TnphoA); FIG. 6C, HrpN_(Ech) ; FIG. 6D, (left side), E. chrysanthemi CUCPB5045 (ΔpelABCE hrpN_(Ech) 546:Tn5-gusA1); 4 (right side), E. chrysanthemi CUCPB5063 (ΔpelABCE outD::TnphoA hrpN_(Ech) 546::Tn5-gusA1).

FIG. 7 shows a diagram of plasmid pCPP19. Significant features are the mobilization (mob) site for conjugation; the cohesive site of λ (cos); and the partition region (par) for stable inheritance of the plasmid. B, BamHI; E, EcoRI; H, HindIII; N, NotI; P, PstI; S. SalI; Sm, SmaI; T3, bacteriophage T3 promoter; T7, bacteriophage T7 promoter; oriV, origin of replication; Sp^(r), spectinomycin resistance; Sm^(r), streptomycin resistance.

DETAILED DESCRIPTION OF THE INVENTION AND DRAWINGS

The present invention relates to an isolated DNA molecule encoding for the hypersensitive response elicitor protein or polypeptide from Erwinia chrysanthemi. For example, this DNA molecule can comprise the nucleotide sequence corresponding to SEQ. ID. No. 1 as follows:

    __________________________________________________________________________     CGATTTTACC CGGGTGAACG TGCTATGACC GACAGCATCA CGGTATTCGA CACCGTTACG    60          - GCGTTTATGG CCGCGATGAA CCGGCATCAG GCGGCGCGCT GGTCGCCGCA ATCCGGCGTC          120                                                                               - GATCTGGTAT TTCAGTTTGG GGACACCGGG CGTGAACTCA TGATGCAGAT TCAGCCGGGG         180                                                                               - CAGCAATATC CCGGCATGTT GCGCACGCTG CTCGCTCGTC GTTATCAGCA GGCGGCAGAG         240                                                                               - TGCGATGGCT GCCATCTGTG CCTGAACGGC AGCGATGTAT TGATCCTCTG GTGGCCGCTG         300                                                                               - CCGTCGGATC CCGGCAGTTA TCCGCAGGTG ATCGAACGTT TGTTTGAACT GGCGGGAATG         360                                                                               - ACGTTGCCGT CGCTATCCAT AGCACCGACG GCGCGTCCGC AGACAGGGAA CGGACGCGCC         420                                                                               - CGATCATTAA GATAAAGGCG GCTTTTTTTA TTGCAAAACG GTAACGGTGA GGAACCGTTT         480                                                                               - CACCGTCGGC GTCACTCAGT AACAAGTATC CATCATGATG CCTACATCGG GATCGGCGTG         540                                                                               - GGCATCCGTT GCAGATACTT TTGCGAACAC CTGACATGAA TGAGGAAACG AAATTATGCA         600                                                                               - AATTACGATC AAAGCGCACA TCGGCGGTGA TTTGGGCGTC TCCGGTCTGG GGCTGGGTGC         660                                                                               - TCAGGGACTG AAAGGACTGA ATTCCGCGGC TTCATCGCTG GGTTCCAGCG TGGATAAACT         720                                                                               - GAGCAGCACC ATCGATAAGT TGACCTCCGC GCTGACTTCG ATGATGTTTG GCGGCGCGCT         780                                                                               - GGCGCAGGGG CTGGGCGCCA GCTCGAAGGG GCTGGGGATG AGCAATCAAC TGGGCCAGTC         840                                                                               - TTTCGGCAAT GGCGCGCAGG GTGCGAGCAA CCTGCTATCC GTACCGAAAT CCGGCGGCGA         900                                                                               - TGCGTTGTCA AAAATGTTTG ATAAAGCGCT GGACGATCTG CTGGGTCATG ACACCGTGAC         960                                                                               - CAAGCTGACT AACCAGAGCA ACCAACTGGC TAATTCAATG CTGAACGCCA GCCAGATGAC         1020                                                                              - CCAGGGTAAT ATGAATGCGT TCGGCAGCGG TGTGAACAAC GCACTGTCGT CCATTCTCGG         1080                                                                              - CAACGGTCTC GGCCAGTCGA TGAGTGGCTT CTCTCAGCCT TCTCTGGGGG CAGGCGGCTT         1140                                                                              - GCAGGGCCTG AGCGGCGCGG GTGCATTCAA CCAGTTGGGT AATGCCATCG GCATGGGCGT         1200                                                                              - GGGGCAGAAT GCTGCGCTGA GTGCGTTGAG TAACGTCAGC ACCCACGTAG ACGGTAACAA         1260                                                                              - CCGCCACTTT GTAGATAAAG AAGATCGCGG CATGGCGAAA GAGATCGGCC AGTTTATGGA         1320                                                                              - TCAGTATCCG GAAATATTCG GTAAACCGGA ATACCAGAAA GATGGCTGGA GTTCGCCGAA         1380                                                                              - GACGGACGAC AAATCCTGGG CTAAAGCGCT GAGTAAACCG GATGATGACG GTATGACCGG         1440                                                                              - CGCCAGCATG GACAAATTCC GTCAGGCGAT GGGTATGATC AAAAGCGCGG TGGCGGGTGA         1500                                                                              - TACCGGCAAT ACCAACCTGA ACCTGCGTGG CGCGGGCGGT GCATCGCTGG GTATCGATGC         1560                                                                              - GGCTGTCGTC GGCGATAAAA TAGCCAACAT GTCGCTGGGT AAGCTGGCCA ACGCCTGATA         1620                                                                              - ATCTGTGCTG GCCTGATAAA GCGGAAACGA AAAAAGAGAC GGGGAAGCCT GTCTCTTTTC         1680                                                                              - TTATTATGCG GTTTATGCGG TTACCTGGAC CGGTTAATCA TCGTCATCGA TCTGGTACAA         1740                                                                              - ACGCACATTT TCCCGTTCAT TCGCGTCGTT ACGCGCCACA ATCGCGATGG CATCTTCCTC         1800                                                                              - GTCGCTCAGA TTGCGCGGCT GATGGGGAAC GCCGGGTGGA ATATAGAGAA ACTCGCCGGC         1860                                                                              - CAGATGGAGA CACGTCTGCG ATAAATCTGT GCCGTAACGT GTTTCTATCC GCCCCTTTAG         1920                                                                              - CAGATAGATT GCGGTTTCGT AATCAACATG GTAATGCGGT TCCGCCTGTG CGCCGGCCGG         1980                                                                              - GATCACCACA ATATTCATAG AAAGCTGTCT TGCACCTACC GTATCGCGGG AGATACCGAC         2040                                                                              - AAAATAGGGC AGTTTTTGCG TGGTATCCGT GGGGTGTTCC GGCCTGACAA TCTTGAGTTG         2100                                                                              - GTTCGTCATC ATCTTTCTCC ATCTGGGCGA CCTGATCGGT T                             2141                                                                           __________________________________________________________________________

Also encompassed by the present invention are fragments of the DNA molecule comprising the nucleotide sequence corresponding to SEQ. ID. No. 1. Suitable fragments capable of eliciting the hypersensitive response (i.e. eliciting necrosis in plants) are constructed by using appropriate restriction sites, revealed by inspection of the DNA molecule's sequence, to: (i) insert an interposon (Felley, et al., "Interposon Mutagenesis of Soil and Water Bacteria: a Family of DNA Fragments Designed for in vitro Insertion Mutagenesis of Gram-negative Bacteria, Gene 52:147-15 (1987), which is hereby incorporated by reference) such that truncated forms of the hypersensitive response elicitor polypeptide or protein, that lack various amounts of the C-terminus, can be produced or (ii) delete various internal portions of the protein. Alternatively, the sequence can be used to amplify any portion of the coding region, such that it can be cloned into a vector supplying both transcription and translation start signals.

Variants may also (or alternatively) be modified by, for example, the deletion or addition of amino acids that have minimal influence on the properties, secondary structure and hydropathic nature of the polypeptide. For example, a polypeptide may be conjugated to a signal (or leader) sequence at the N-terminal end of the protein which co-translationally or post-translationally directs transfer of the protein. The polypeptide may also be conjugated to a linker or other sequence for ease of synthesis, purification or identification of the polypeptide.

The DNA molecule, corresponding to SEQ. ID. No. 1, contains an open reading frame which codes for the hypersensitive response elicitor polypeptide or protein from Erwinia chrysanthemi and corresponds to SEQ. ID. No. 2 as follows:

    __________________________________________________________________________     Met Gln Ile Thr Ile Lys Ala His Ile Gly Gly Asp Leu Gly Val Ser                  1               5                   10                  15                      - Gly Leu Gly Ala Gln Gly Leu Lys Gly Leu Asn Ser Ala Ala Ser Ser                        20                  25                  30                          - Leu Gly Ser Ser Val Asp Lys Leu Ser Ser Thr Ile Asp Lys Leu Thr                     35                  40                  45                              - Ser Ala Leu Thr Ser Met Met Phe Gly Gly Ala Leu Ala Gln Gly Leu                 50                  55                  60                                  - Gly Ala Ser Ser Lys Gly Leu Gly Met Ser Asn Gln Leu Gly Gln Ser             65                  70                  75                  80                  - Phe Gly Asn Gly Ala Gln Gly Ala Ser Asn Leu Leu Ser Val Pro Lys                             85                  90                  95                      - Ser Gly Gly Asp Ala Leu Ser Lys Met Phe Asp Lys Ala Leu Asp Asp                         100                 105                 110                         - Leu Leu Gly His Asp Thr Val Thr Lys Leu Thr Asn Gln Ser Asn Gln                     115                 120                 125                             - Leu Ala Asn Ser Met Leu Asn Ala Ser Gln Met Thr Gln Gly Asn Met                 130                 135                 140                                 - Asn Ala Phe Gly Ser Gly Va1 Asn Asn Ala Leu Ser Ser Ile Leu Gly             145                 150                 155                 160                 - Asn Gly Leu Gly Gln Ser Met Ser Gly Phe Ser Gln Pro Ser Leu Gly                             165                 170                 175                     - Ala Gly Gly Leu Gln Gly Leu Ser Gly Ala Gly Ala Phe Asn Gln Leu                         180                 185                 190                         - Gly Asn Ala Ile Gly Met Gly Val Gly Gln Asn Ala Ala Leu Ser Ala                     195                 200                 205                             - Leu Ser Asn Val Ser Thr His Val Asp Gly Asn Asn Arg HIs Phe Val                 210                 215                 220                                 - Asp Lys Glu Asp Arg Gly Met Ala Lys Glu Ile Gly Gln Phe Met Asp             225                 230                 235                 240                 - Gln Tyr Pro Glu Ile Phe Gly Lys Pro Glu Tyr Gln Lys Asp Gly Trp                             245                 250                 255                     - Ser Ser Pro Lys Thr Asp Asp Lys Ser Trp Ala Lys Ala Leu Ser Lys                         260                 265                 270                         - Pro Asp Asp Asp Gly Met Thr Gly Ala Ser Met Asp Lys Phe Arg Gln                     275                 280                 285                             - Ala Met Gly Met Ile Lys Ser Ala Val Ala Gly Asp Thr Gly Asn Thr                 290                 295                 300                                 - Asn Leu Asn Leu Arg Gly Ala Gly Gly Ala Ser Leu Gly Ile Asp Ala             305                 310                 315                 320                 - Ala Val Val Gly Asp Lys Ile Ala Asn Met Ser Leu Gly Lys Leu Ala                             325                 330                 335                     - Asn Ala                                                                    __________________________________________________________________________

This protein or polypeptide has a molecular weight of 32 to 36 kDa, preferably 34 kDa. It is heat stable (i.e., activity is retained upon boiling for 10 min.), has a glycine content of greater than 16%, and contains no cysteine.

The DNA molecule containing just the open reading frame coding for SEQ. ID. No. 2 has the nucleotide sequence corresponding to SEQ. ID. NO. 6 as follows:

    __________________________________________________________________________       1  ATGCAAATTACGATCAAAGCGCACATCGGCGGTGATTTGGGCGTCTCCGGTCTGGGGCTG    60          -  61  GGTGCTCAGGGACTGAAAGGACTGAATTCCGCGGCTTCATCGCTGGGTTCCAGCGTGGAT          120                                                                               -  121  AAACTGAGCAGCACCATCGATAAGTTGACCTCCGCGCTGACTTCGATGATGTTTGGCGGC        180                                                                              -  181  GCGCTGGCGCAGGGGCTGGGCGCCAGCTCGAAGGGGCTGGGGATGAGCAATCAACTGGGC         240                                                                               -  241  CAGTCTTTCGGCAATGGCGCGCAGGGTGCGAGCAACCTGCTATCCGTACCGAAATCCGGC        300                                                                              -  301  GGCGATGCGTTGTCAAAAATGTTTGATAAAGCGCTGGACGATCTGCTGGGTCATGACACC         360                                                                               -  361  GTGACCAAGCTGACTAACCAGAGCAACCAACTGGCTAATTCAATGCTGAACGCCAGCCAG        420                                                                              -  421  ATGACCCAGGGTAATATGAATGCGTTCGGCAGCGGTGTGAACAACGCACTGTCGTCCATT         480                                                                               -  481  CTCGGCAACGGTCTCGGCCAGTCGATGAGTGGCTTCTCTCAGCCTTCTCTGGGGGCAGGC        540                                                                              -  541  GGCTTGCAGGGCCTGAGCGGCGCGGGTGCATTCAACCAGTTGGGTAATGCCATCGGCATG         600                                                                               -  601  GGCGTGGGGCAGAATGCTGCGCTGAGTGCGTTGAGTAACGTCAGCACCCACGTAGACGGT        660                                                                              -  661  AACAACCGCCACTTTGTAGATAAAGAAGATCGCGGCATGGCGAAAGAGATCGGCCAGTTT         720                                                                               -  721  ATGGATCAGTATCCGGAAATATTCGGTAAACCGGAATACCAGAAAGATGGCTGGAGTTCG        780                                                                              -  781  CCGAAGACGGACGACAAATCCTGGGCTAAAGCGCTGAGTAAACCGGATGATGACGGTATG         840                                                                               -  841  ACCGGCGCCAGCATGGACAAATTCCGTCAGGCGATGGGTATGATCAAAAGCGCGGTGGCG        900                                                                              -  901  GGTGATACCGGCAATACCAACCTGAACCTGCGTGGCGCGGGCGGTGCATCGCTGGGTATC         960                                                                               -  961  GATGCGGCTGTCGTCGGCGATAAAATAGCCAACATGTCGCTGGGTAAGCTGGCCAACGCC        1020                                                                              - 1021  TGA                                                                 1023                                                                           __________________________________________________________________________

The protein or polypeptide of the present invention is preferably produced in purified form (preferably, at least: about 80%, more preferably 90%, pure) by conventional techniques. Typically, the protein or polypeptide of the present invention is isolated from E. coli by lysing and sonication. After washing, the lysate pellet is resuspended in buffer containing Tris-HCl. During dialysis, a precipitate forms from this protein solution. The solution is centrifuged, and the pellet is washed and resuspended in the buffer containing Tris-HCl. Proteins are resolved by electrophoresis through an SDS 12% polyacrylamide gel.

The DNA molecule C encoding the hypersensitive response elicitor polypeptide from Erwinia chrysanthemi can be incorporated in cells using conventional recombinant DNA technology. Generally, this involves inserting the DNA molecule into an expression system to which the DNA molecule is heterologous (i.e. not normally present). The heterologous DNA molecule is inserted into the expression system or vector in proper sense orientation and correct reading frame. The vector contains the necessary elements for the transcription and translation of the inserted protein-coding sequences.

U.S. Pat. No. 4,237,224 to Cohen and Boyer, which is hereby incorporated by reference, describes the production of expression systems in the form of recombinant plasmids using restriction enzyme cleavage and ligation with DNA ligase. These recombinant plasmids are then introduced by means of transformation and replicated in unicellular cultures including procaryotic organisms and eucaryotic cells grown in tissue culture.

Recombinant genes may also be introduced into viruses, such as vaccina virus. Recombinant viruses can be generated by transfection of plasmids into cells infected with virus.

Suitable vectors include, but are not limited to, the following viral vectors such as lambda vector system gt11, gt WES.tB, Charon 4, and plasmid vectors such as pER322, pBR325, pACYC177, pACYC184, pUC8, pUC9, pUC18, pUC19, pLG339, pR290, pKC37, pKC101, SV 40, pBluescript II SK +/- or KS +/- (see "Stratagene Cloning Systems" Catalog (1993) from Stratagene, La. Jolla, Calif., which is hereby incorporated by reference), pQE, pIH821, pGEX, pET series (see F. W. Studier et. al., "Use of T7 RNA Polymerase to Direct Expression of Cloned Genes," Gene Expression Technology vol. 185 (1990), which is hereby incorporated by reference), and any derivatives thereof. Recombinant molecules can be introduced into cells via transformation, transduction, conjugation, mobilization, or electroporation. The DNA sequences are cloned into the vector using standard cloning procedures in the art, as described by Maniatis et al., Molecular Cloning: A Laboratory Manual, Cold Springs Laboratory, Cold Springs Harbor, N.Y. (1982), which is hereby incorporated by reference.

A variety of host-vector systems may be utilized to express the protein-encoding sequence(s). Primarily, the vector system must be compatible with the host cell used. Host-vector systems include but are not limited to the following: bacteria transformed with bacteriophage DNA, plasmid DNA, or cosmid DNA; microorganisms such as yeast containing yeast vectors; mammalian cell systems infected with virus (e.g., vaccinia virus, adenovirus, etc.); insect cell systems infected with virus (e.g., baculovirus); and plant cells infected by bacteria or transformed via particle bombardment (i.e. biolistics). The expression elements of these vectors vary in their strength and specificities. Depending upon the host-vector system utilized, any one of a number of suitable transcription and translation elements can be used.

Different genetic signals and processing events control many levels of gene expression (e.g., DNA transcription and messenger RNA (mRNA) translation).

Transcription of DNA is dependent upon the presence of a promotor which is a DNA sequence that directs the binding of RNA polymerase and thereby promotes MRNA synthesis. The DNA sequences of eucaryotic promotors differ from those of procaryotic promoters. Furthermore, eucaryotic promotors and accompanying genetic signals may not be recognized in or may not function in a procaryotic system, and, further, procaryotic promotors are not recognized and do not function in eucaryotic: cells.

Similarly, translation of mRNA in procaryotes depends upon the presence of the proper procaryotic signals which differ from those of eucaryotes. Efficient translation of mRNA in procaryotes requires a ribosome binding site called the Shine-Dalgarno ("SD") sequence on the MRNA. This sequence is a short nucleotide sequence of MRNA that is located before the start codon, usually AUG, which encodes the amino-terminal methionine of the protein. The SD sequences are complementary to the 3'-end of the 16S rRNA (ribosomal RNA) and probably promote binding of mRNA to ribosomes by duplexing with the rRNA to allow correct positioning of the ribosome. For a review on maximizing gene expression, see Roberts and Lauer, Methods in Enzymology, 68:473 (1979), which is hereby incorporated by reference.

Promotors vary in their "strength" (i.e. their ability to promote transcription). For the purposes of expressing a cloned gene, it is desirable to use strong promoters in order to obtain a high level of transcription and, hence, expression of the gene. Depending upon the host cell system utilized, any one of a number of suitable promotors may be used. For instance, when cloning in E. coli, its bacteriophages, or plasmids, promoters such as the T7 phage promoter, lac promotor, trp promotor, reca promotor, ribosomal RNA promotor, the P_(R) and P_(L) promotors of coliphage lambda and others, including but not limited, to lacUV5, ompF, bla, lpp, and the like, may be used to direct high levels of transcription of adjacent DNA segments. Additionally, a hybrid trp-lacUV5 (tac) promotor or other E. coli promoters produced by recombinant DNA or other synthetic DNA techniques may be used to provide for transcription of the inserted gene.

Bacterial host cell strains and expression vectors may be chosen which inhibit the action of the promotor unless specifically induced. In certain operons, the addition of specific inducers is necessary for efficient transcription of the inserted DNA. For example, the lac operon is induced by the addition of lactose or IPTG (isopropylthio-beta-D-galactoside). A variety of other operons, such as trp, pro, etc., are under different controls.

Specific initiation signals are also required for efficient gene transcription and translation in procaryotic cells. These transcription and translation initiation signals may vary in "strength" as measured by the quantity of gene specific messenger RNA and protein synthesized, respectively. The DNA expression vector, which contains a promotor, may also contain any combination of various "strong" transcription and/or translation initiation signals. For instance, efficient translation in E. coli requires a Shine-Dalgarno (SD) sequences about 7-9 bases 5' to the initiation codon (ATG) to provide a ribosome binding site. Thus, any SD-ATG combination that can be utilized by host cell ribosomes may be employed. Such combinations include but are not limited to the SD-ATG combination from the cro gene or the N gene of coliphage lambda, or from the E. coli tryptophan E, D, C, B or A genes. Additionally, any SD-ATG combination produced by recombinant DNA or other techniques involving incorporation of synthetic nucleotides may be used.

Once the isolated DNA molecule encoding the hypersensitive response elicitor protein or polypeptide from Erwinia chrysanthemi has been cloned into an expression system, it is ready to be incorporated into a host cell. Such incorporation can be carried out by the various forms of transformation noted above, depending upon the vector/host cell system. Suitable host cells include, but are not limited to, bacteria, virus, yeast, mammalian cells, insect, plant, and the like.

One aspect of the present invention involves using the DNA molecule encoding the hypersensitive response elicitor from Erwinia chrysanthemi to transform plants in order to impart localized resistance to pathogens, such as bacteria, fungi, nematodes, and viruses, using a promoter specifically activated by one or more of these pathogens. As a result, when the transgenic plant is infiltrated by such pathogens, the leaves of the plant will undergo localized collapse. This confines the pathogen so that further growth is prevented, and, ultimately, the pathogen perishes. Transformation of plants with the DNA molecule of the present invention is particularly useful where the plant does not exhibit a hypersensitive response to pathogens or is weakly responsive to such pathogens. This requires that hrpN_(ech) be hooked up to the promotor of a plant gene that the pathogen induces such as PAL, CHS, etc. Otherwise, hrpN will kill the plant. It may also be appropriate to transform plants with this DNA molecule where the pathogens they are susceptible to do not elicit a hypersensitive response or elicit a response which is weak.

This aspect of the present invention requires that the subject DNA molecule be incorporated in plants with a suitable promoter which is activated by pathogen infection. Suitable promoters for these purposes include those from the following genes: genes expressed in response to fungus and bacterial infection (e.g., genes encoding phenylalanine ammonia lyase and chalcone synthase) and genes involved in the development of senescence.

The isolated DNA molecule of the present invention can be utilized to impart localized resistance to pathogens for a wide variety of plants, including both monocots and dicots. Although the gene can be inserted into any plant falling within these broad classes, it is particularly useful in crop plants, such as rice, wheat, barley, rye, corn, potato, sweet potato, bean, pea, chicory, lettuce, endive, cabbage, cauliflower, broccoli, turnip, radish, spinach, asparagus, onion, garlic, eggplant, pepper, celery, carrot, squash, pumpkin, zucchini, cucumber, apple, pear, quince, melon, plum, cherry, peach, nectarine, apricot, strawberry, grape, raspberry, blackberry, pineapple, avocado, papaya, mango, banana, soybean, tobacco, tomato, sorghum, and sugarcane. The present invention may also be used in conjunction with non-crop plants, including Arabidopsis thaliana, saintpaulia, petunia, pelargonium, and zinnia.

The expression system of the present invention can also be used to transform virtually any plant cell under suitable conditions. Cells transformed in accordance with the present invention can be grown in vitro in a suitable medium to impart localized resistance to pathogens. Transformed cells can be regenerated into whole plants such that this protein imparts localized resistance to pathogens to the intact transgenic plants. In either case, the plant cells transformed with the recombinant DNA expression system of the present invention are grown and caused to express that DNA in the cells in response to a pathogen to impart localized resistance to pathogens on them.

One technique of transforming plants with the DNA molecule in accordance with the present invention is by contacting the tissue of such plants with an inoculum of a bacteria transformed with a vector comprising a gene in accordance with the present invention which imparts localized resistance to pathogens. Generally, this procedure involves inoculating the plant tissue with a suspension of bacteria and incubating the tissue for 48 to 72 hours on regeneration medium without antibiotics at 25-28° C.

Bacteria from the genus Agrobacterium can be utilized to transform plant cells. Suitable species of such bacterium include Agrobacterium tumefaciens and Agrobacterium rhizogenes. Agrobacterium tumefaciens (e.g., strains LBA4404 or EHA105) is particularly useful due to its well-known ability to transform plants.

Another approach to transforming plant cells with a gene which imparts localized resistance to pathogens is particle bombardment (also known as biolistic transformation) of the host cell. This can be accomplished in one of several ways. The first involves propelling inert or biologically active particles at cells. This technique is disclosed in U.S. Pat. Nos. 4,945,050, 5,036,006, and 5,100,792, all to Sanford et al., which are hereby incorporated by reference. Generally, this procedure involves propelling inert or biologically active particles at the cells under conditions effective to penetrate the outer surface of the cell and to be incorporated within the interior thereof. When inert particles are utilized, the vector can be introduced into the cell by coating the particles with the vector containing the heterologous DNA. Alternatively, the target cell can be surrounded by the vectcr so that the vector is carried into the cell by the wake of the particle. Biologically active particles (e.g., dried bacterial cells containing the vector and heterologous DNA) can also be propelled into plant cells.

The hypersensitive response elicitor polypeptide or protein of the present invention can be applied to all or part: of the plants listed previously to impart resistance to the plants. In this procedure, the polypeptide or protein alone, mixed with a carrier, and/or with plant treating agents (e.g., fertilizers, insecticides, fungicides, etc.) can be applied to plant. Such application requires infiltration of the polypeptide or protein into the plant by, for example, injection, leaf abrasion proximate to the time of application, or high pressure spraying.

The isolated DNA molecule of the present invention could be used to generate male sterile plants used in plant breeding for generation of hybrids. A promoter from a plant gene involved specifically in pollen development could be useds to express the hypersensitive response elicitor polypeptide or protein in anthers, resulting in death of the anther and/or pollen.

The hypersensitive response elicitor polypeptide or protein of the present invention could be used as a regulatable suicide factor to kill a transgenic plant on demand. If it were engineered to be expressed from a promoter that is induced in response to a normally nontoxic, environmentally friendly, inducer molecule, the plant could be killed by spraying with the inducer molecule rather than with herbicides. This could be used, for example, instead of herbicides to kill potato vines to facilitate harvest and reduce the devastating tuber blight phase of late blight. This could also be linked with other transgenes to permit the control of a transgenic plant that escaped or outcrossed with a weed. For example, a concern with pathogen-derived resistance to viruses is that the virus resistance genes could spread from the transgenic crop to related weeds that are normally held in check by natural virus infections. These new, more hardy weeds, would now be sensitive to killing by the inducer of the gene.

The hypersensitive response elicitor polypeptide or protein of the present invention may be used as a selective herbicide in synergistic combination with an avirulence protein that interacts with a resistance gene product that is unique t:-o the targeted weed (or is lacking from crop plants).

It may be possible to use the hypersensitive response elicitor polypeptide or protein of the present invention in a tissue culture selection scheme to select cultures that are resistant to it. Regenerated plants may then be resistant to E. chrysanthermi and may even be resistant to a wide range of plant pathogenic bacteria. It is possible that this protein or polypeptide has effects on animal cells that could be exploited for medical use, insect control.

The hypersensitive response elicitor protein or polypeptide can also be used to raise monoclonal or polyclonal antibodies by conventional procedures. At least the binding portions of these antibodies can be sequenced and encoding DNA synthesized. The encoding DNA molecule can be used to transform plants together with a promoter which causes expression of the encoded protein when the plant is infected by Erwinia chrysanthemi. The expressed protein will bind to the pathogen and help prevent the usual necrotic response. Antibodies to this protein or polypeptide of the present invention could also be used to identify pathogenic Erwinia.

EXAMPLES Example 1 Bacterial Strains, Plasmids and Culture Conditions

Bacterial strains and plasmids are listed in Table 1.

                  TABLE 1                                                          ______________________________________                                         Bacterial strains and plasmids utilized                                                       Relevant                                                          Designation characteristics.sup.a Reference or source                        ______________________________________                                         Escherichia coli                                                                          supE44 supF58 hsdS3 (r.sub.B.sup.-                                                             (Sambrook et al.                                      ED8767 m.sub.B.sup.-) 1989).sup.b                                               recA56 galK2 galT22                                                            metB1                                                                         DH5α supE44 ΔlacU169 (φ80 (Hanahan 1983).sup.c ;                                          lacZΔM15) hsdR17 recA1 Life Technologie                                s,                                                     endA1 gyrA95 thi-1 Inc. (Grand Island,                                         relA1 Nal.sup.r NY)                                                           DH10B mcrA Δ(mrr-hsdRMS- (Grant et al.                                    mcrBC) φ80 lacZΔM15 1990).sup.d ; Life                               ΔlacX74 deoR recA1 Technologies, Inc.                                    endA1 araD139 Δ(ara,                                                     leu) 7697 galU galK                                                            rpsL nupG                                                                     Erwinia Wild-type strain (Burkholder et al.                                    chrysanthemi  1953).sup.e                                                      EC16                                                                           AC4150 Spontaneous Nal.sup.r (Chatterjee et al.                                 derivative of EC16 1983).sup.f                                                CUCPB5006 Δ(pelB peC)::28bp (He and Collmer                               Δ(pelA pelE) derivative 1990).sup.g                                      of AC4150                                                                     CUCPB5030 outD::TnphoA derivative (Bauer et al.                                 of CUCPB5006 1994).sup.h                                                      CUCPB5045 hrpN.sub.Ech 546::Tn5-gusA1 Described in this                         derivative of CUCPB5006 application                                           CUCPB5046 hrpN.sub.Ech 439::Tn5-gusA1 Described in this                         derivative of CUCPB5006 application                                           CUCPB5063 hrpN.sub.Ech 546::Tn5-gusA1 Described in this                         derivative of CUCPB5030 application                                           CUCPB5049 hrpN.sub.Ech :439::Tn5-gusA1 Described in this                        derivative of AC4150.sup.+ application                                        Erwinia amylovora                                                              Ea321 Wild type ATCC 49947; CNPB                                                 1367                                                                         Ea321T5 hrpN.sub.Ea ::Tn5tac1 (Wei et al. 1992).sup.i                           derivative of Ea321                                                           Plasmids and                                                                   phage                                                                          pBluescript II Amp.sup.r Stratagene (La                                        SK(-)  Jolla CA)                                                               pCCP19 Cosmid Vector, Sp.sup.r /Sm.sup.r See FIG. 7.                           pUC119 Amp.sup.r plasmid vector (Vieira and Messing                              1987).sup.j                                                                  pSE280 Amp.sup.r plasmid vector with (Brosius 1989).sup.k                       superpolylinker                                                                downstream of tac                                                              promoter                                                                      pCPP2030 pCPP19 carrying E. (Bauer et al.                                       chrysanthemi DNA 1994).sup.h                                                   hybridizing with E.                                                            amylovora hrp genes in                                                         pCPP1033                                                                      pCPP1084 pBluescript M13+ (Wei et al. 1992).sup.i                               carrying hrpN.sub.Ea on 1.3                                                    kb Hindlll fragment                                                           pCPP2157 pCPP19 carrying E. Described in this                                   chrysanthemi DNA application                                                   hybridizing with E.                                                            amylovora hrpN                                                                pCPP2142 8.3 kb SalI subclone Described in this                                 from pCPP2157 in pUC119 application                                           pCPP2141 3.1 kb Pstl subclone Described in this                                 from pCPP2157 in application                                                   pBluescript II SK(-)                                                           hrpN.sub.Ech in opposite                                                       orientation from vector                                                        lac promoter                                                                  pCPP2172 3.1 kb Pstl subclone Described in this                                 from pCPP2157 in application                                                   pBluescript II SK(-)                                                           hrpN.sub.Ech in same                                                           orientation as vector                                                          lac promoter                                                                  pCPP2174 1.0 kb hrpN.sub.Ech + PCR Described in this                            product cloned in NcoI- application                                            HindIII sites of pSE280                                                       ::Tn5-gusA1 Tn5 derivative for (Sharma and Signer                               generating 1990).sup.1                                                         transciptional fusions                                                         with uidA receptor;                                                            Kan.sup.r, Tet.sup.r                                                        ______________________________________                                          .sup.a Amp.sup.r = ampicillin resistance: Nal.sup.r = nalidixic acid           resistance; Sm.sup.r = streptomycin resistance; Sp.sup.r = spectinomycin       resistance; Tet.sup.r = tetracycline resistance                                .sup.b Sambrook, J., Fritsch, E. F., and Maniatis, T., Molecular Cloning.      A laboratory Manual, Second Edition. Cold Spring Harbor, Cold Spring           Harbor (1989), which is hereby incorporated by reference.                      .sup.c Hanahan, D., "Studies on Transformation of Escherichia coli with        Plasmids," J. Mol. Biol. 166: 557-580 (1983), which is hereby incorporate      by reference.                                                                  .sup.d Grant, S. G. N., Jessee, J., Bloom, F. R., and Hanahan, D.,             "Differential Plasmid, Rescue from Transgenic Mouse DNAs in Escherichia        coli Methylationrestriction Mutants," Proc. Nat. Acad. Sci. U.S.A.             87465-46949 (1990), which is hereby incorporated by reference.                 .sup.e Burkholder, W, H., McFadden, L. A., and Dimock, A. W, "A Bacterial      Blight of Chrysanthemums," Phytopathology 43: 522-526 (1953), which is         hereby incorporated by reference.                                              .sup.f Chatterjee, A. K., Thurn, K. K., and Feese, D. A." Tn5 Induced          Mutations in the Enterobacterial Phytopathogen Erwinia chrysanthemi,"          Appl. Environ. Microbiol. 45: 644-650 (1983), which is hereby incorporate      by reference.                                                                  .sup.g He, S. Y., and Collmer, A, "Molecular Cloning, Nucleotide Sequence      and Markerexchange Mutagenesis of the Exopoly-D-galacturonosidase-encodin      pehX Gene of Erwinia chrysanthemi EC16," J. Bacterial. 172: 4988-4995          (1990), which is hereby incorporated by reference.                             .sup.h Bauer, D. W., Bogdanove, A. J., Beer, S. V., and Collmer, A.,           "Erwinia Chrysanthemi hrp Genes and their Involvement in Soft Rot              Pathogenesis and Elicitation of the Hypersensitive Response," Mol.             PlantMicrobe Interact., 7: 573-581 (1994), which is hereby incorporated b      reference.                                                                     .sup.i Wei, Z. M., Laby, R. J., Zumoff, C. H., Bauer, D. W., He, S. Y.,        Collmer, A., and Beer, S. V., "Harpin, Elicitor of the Hypersensitive          Response Produced by the Plant Pathogen Erwinia amylovora," Science 257:       85-88 (1992), which is hereby incorporated by reference.                       .sup.j Vieira, J., and Messing, J., "Production of Singlestranded Plasmid      DNA," Meth. Enzymol. 153: 3-11 (1987), which is hereby incorporated by         reference.                                                                     .sup.k Brocius, J., "Superpolylinkers in Cloning and Expression Vectors,"      DNA 8: 759-777 (1989), which is hereby incorporated by reference.              .sup.l Sharma, S. B., and Signer, E. R., "Temporal and Spatial Regulation      of the Symbiotic Genes of Rhizobium meliloti in Planta Revealed by             Transposon Tn5gusA," Genes Develop. 4: 344-356 (1990), which is hereby         incorporated by reference.                                               

E. chrysanthemi was routinely grown in King's medium B ("KB") (King, E. O. et al., "Two Simple Media for the Demonstration of Pyocyanin and Fluorescein," J. Lab. Med. 22:301-307 (1954), which is hereby incorporated by reference) at 30° C., E. coli in LM medium (Hanahan, D., "Studies on transformation of Escherichia coli with Plasmids," J. Mol. Biol. 166:557-580 (1983), which is incorporated by reference) at 37° C., and E. amylovora in LB medium at 28-30° C. The following antibiotics were used in selective media in the amounts indicated (μg/ml) except where noted: ampicillin, 100; kanamycin, 50; spectinomycin, 50; and streptomycin, 25.

Example 2 General DNA Manipulations

Plasmid DNA manipulations, colony blotting, and Southern blot analyses were performed using standard techniques (Sambrook, J., et al., "Molecular Cloning. A Laboratory Manual," Second Edition, Cold Spring Harbor, Cold Spring Harbor (1989), which is incorporated by reference). Deletions for sequencing were constructed with the Erase-a-Base Kit (Promegsa, Madison, Wis.). Double stranded DNA sequencing templates were prepared using Qiagen Plasmid Mini Kits (Chatsworth, Calif.). Sequencing was performed using the Sequenase Version 2 kit (U.S. Biochemical, Cleveland, Ohio). The TnS-gusA1 insertion points were determined on an Applied Biosystems (Foster City, Calif.) Automated DNA Sequencer Model 373A by the Cornell Biotechnology Center. DNA sequences were analyzed with the Genetics Computer Group Sequence Analysis Software Package (Devereaux, J., et al., "A Comprehensive Set of Sequence Analysis Programs for the VAX," Gene 12:387-395 (1984), which. is hereby incorporated by reference). Comparison of HrpN_(Ech) (i.e. the gene encoding the hypersensitive response elicitor polypeptide or protein from Erwinia chrysanthemi) and HrpN_(Ea) (i.e. the gene encoding the hypersensitive response elicitor polypeptide or protein from Erwinia amylovora) by the Gap Program was done with a gap weight of 5.0 and a gap length weight of 0.3. Marker-exchange mutagenesis was performed as described (Bauer, D. W., et al., "Erwinia chrysanthemi hzp Genes and Their Involvement in Soft Rot Pathogenesis and Elicitation of the Hypersensitive Response," Mol. Plant-Microbe Interact, 7:573-581 (1994), which is incorporated by reference). The oligonucleotide used to determine the location of Tn5-gusA1 insertions in hrpN_(Ech) was TGACCTGCAGCCAAGCTTTCC (SEQ. ID. No. 3). The oligonucleotide used as the first primer to amplify the hrpN_(Ech) ORF and to introduce a Ncol site at the 5' end of the gene was AGTACCATGGTTATTACGATCAAAGCGCAC (SEQ. ID. No. 4); the one used as the second primer to introduce an Xhol site at the 3' end of the gene was AGATCTCGAGGGCGTTGGCCAGCTTl(CC (SEQ. ID. No. 5). Primers were synthesized by Integrated DNZI Technologies (Coralville, Iowa).

Example 3 Protein Manipulations

HrpN_(Ech) was purified from E. coli Dh5α(pCPP2172) cultures grown at 30° C. to stationary phase in 50 ml of Terrific Broth (Sambrook, J., et al., "Molecular Cloning. A Laboratory Manual," Second Edition, Cold Spring Harbor, Cold Spring Harbor (1989), which is incorporated by reference) supplemented withl ampicillin at a concentration of 200μg/ml. fells were lysed by lysozyme treatment and sonication as described (Sambrook J., et al., "Molecular Cloning. A Laboratory Manual," Second Edition, Cold Spring Harbor, Cold Spring Harbor (1989), which is incorporated by reference). The lysate pellet was washed twice with 9 volumes of lysis buffer containing 0.5% Triton X-100 and 10 mM EDTA, pH 8.0, 0.1 mM phenylmethylsulfonyl fluoride (PMSF), and reharvested each time by centrifugation at 12,000× g for 15 min. The pellet was resuspended in 2.0 ml lysis buffer containing 0.1 mM PMSF, dissolved by the addition of 2.5 ml of 8 M guanidine-HCl in lysis buffer and then diluted with 5.0 ml water. The protein solution was dialyzed in SpectraPor #1 dialysis tubing against 2.0 I of 5 mM MES, pH 6.5, containing 0.05 mM PMSF. The precipitate that formed during dialysis and the solution were centrifuged for 15 min at 4,300× g. The pellet was washed once with 10 ml of 5 mM MES, pH. 6.5, with 0.1 mM PMSF and then resuspended in 2.0 ml of the same buffer. Protein concentrations of homogeneous suspensions were determined following dissolution in the reagents of the dye-binding assay of Bradford (Bradford, M., "A Rapid and Sensitive Method for the Quantitation of Microgram Quantities of Protein Utilizing the Principle of Protein-dye-binding," Anal. Biochem, 92:248-2254 (1976), which is incorporated by reference). Proteins in crude cell lysates or following purification were resolved by electrophoresis through an SDS 120 polyacrylamide gel and visualized by staining with Coomassie Brilliant Blue R. The N-terminal sequence of purified HrpN_(Ech) was determined at the Cornell University Biotechnology Program Analysis Facility.

Example 4 Plant Assays

Tobacco (Nicotiana tabacum L. var Xanthi), tomato (Lycopersicon esculentum Mill. var Sweet 199), pepper (Capsicum annuum L. var Sweet Hungarian), Saintpaulia (S. ionantha Wendl. var. Paris), petunia (P. grandiflora Juss. var. Blue Frost), pelargonium (P. hortorum Bailey), winter squash (Cucurbita maxima Duchesne.), and Zinnia (Z. elegans Jacq.) plants were grown under greenhouse conditions or purchased at a local garden shop and then maintained in the laboratory at room temperature, with incident daylight supplemented with a 500 W halogen lamp, for hypersensitive response assays. Witloof chicory (Cichorium intybus L.) "IBelgian endive" heads were purchased from a local supermarket. Bacterial inoculum was prepared and delivered as previously described (Bauer, D. W., et al., "Erwinia chrysanthemi hrp Genes and Their Involvement in S Soft Rot Pathogenesis and Elicitation of the Hypersensitive Response," Mol. Plant-Microbe Interact, 7:573-581 (1994) which is incorporated by reference). Briefly, to assay soft-rot pathogenesis, 5 μl of inoculum was applied to a small wound in detached chicory leaves; to assay for hypersensitive response elicitation, inoculum was injected into the intercellular spaces of plant leaves with a needle-less plastic syringe. Hereafter, in these examples, this injection procedure is referred to as "infiltrated" or as "infiltration".

Example 5 Molecular Cloning of the E. chrysanthemi hrpN_(Ech) Gene

Eighteen cosmids containing E. chrysanthemi DNA sequences hybridizing with a region of the E. amylovora hrp cluster that is widely conserved in plant pathogenic bacteria. were previously isolated (Bauer, D. W., et al., "Erwinia chrysanthemi hrp Genes and Their Involvement in Soft Rot Pathogenesis and Elicitation of the Hypersensitive Response," Mol. Plant-Microbe Interact, 7:573-581 (1994), which is incorporated by reference). The pattern of restriction fragments released from these cosmids indicated they all contained overlapping inserts from the same region of the E. chrysanthemi genome. The cosmids were probed in colony blots with a 1.3 kb Hindlll fragment from pCPP1084, which contains the E. amylovora hrpN gene (Wei, Z.-M., et al., "Harpin Elicitor of the Hypersensitive Response Produced by the Plant: Pathogen," Science 257:85-88 (1992), which is incorporated by reference). pCPP2157, one of the three cosmids hybridizing with the probe, was digested with several restriction enzymes, and the location of the hrpN_(Ech) gene in those fragments was determined by probing a Southern blot with the E. amylovora Hindlll fragment. Two fragments, each containing the entire hrpN_(Ech) gene, were subcloned into different vectors: pCPP2142 contained an 8.3 kb SalI fragment in pUC119 (Vieira, J., et al., "Production of Single-stranded Plasmid DNA, Meth. Enzymo, 153:3-11 (1987), which is incorporated by reference), and pCPP2141 contained a 3.1 kb Pstl fragment in pBluescript II SK(-) (Stratagene, La Jolla, Calif.) and was used to sequence the hrpN_(Ech) gene.

Example 6 Sequence of hrpN_(Ech)

The nucleotide sequence of a 2.4 kb region of pCPP2141 encompassing hrpN_(Ech) was determined. The portion of that sequence extending from the putative ribosome binding site through the hrpN_(Ech) coding sequence to a putative rho-independent terminator is presented in FIG. 1. The typical ribosome-binding site, consisting of GAGGA, was located 10 bases upstream of the ATG translational initiation codon. No promoter sequences were discernible upstream of hrpN_(Ech). Instead, the presence of another open reading frame suggested that hrpN_(Ech) is the last open reading frame in a polycistronic operon. hrpN_(Ech) codes for a predicted protein that is 34.3 kD, rich in glycine (16.2%) and lacking in cysteine. Comparison of the amino acid sequences of the predicted hrpN_(Ea) and hrpN_(Ech) products revealed extensive similarity, particularly in the C-terminal halves of the proteins (FIG. 2). The overall identity of the hrpN genes and proteins was 66.9% and 45.5%, as determined by the FASTA and Gap algorithms, respectively (Devereaux, J., et al., "A Comprehensive Set of Sequence Analysis Programs for the VAX," Gene 12:387-395 (1984); Pearson, W. R., et al., "Improved Tools for Biological Sequence Comparison," Proc. Natl. Acad. Sci. U.S.A., 85:2444-2448 (1988), which are hereby incorporated by reference).

The direction of hrpN_(Ech) transcription, the size of the predicted product, and the translation start site were confirmed by recloning the 3.1 kb Pstl fragment from pCPP2157 and selecting a clone with the fragment in pbluescript II SK(-) in the opposite orientation from pCPP2141 to produce pCPP2172. E. coli DH5α(pCPP2172) expressed hrpN_(Ech) from the vector lac promoter and produced high levels of a protein with an estimated molecular mass of 36 kD in SDS polyacrylamide gels, which is close to the predicted size (FIG. 3). Furthermore, the N-terminal 10 amino acids of the 36 kD protein, determined by microsequencing following purification as described below, corresponded with the predicted N-terminus of HrpN_(Ech). No N-terminal signal sequence for targeting to the general export (Sec) pathway was discernible in the HrpN_(Ech) sequence, and the data showed no evidence of processing of the N-terminus.

Example 7 Purification of the hrpN_(Ech) Product and Demonstration of its Hypersensitive Response Elicitor Activity in Tobacco

When DH5α(pCPP2172) cells were disrupted by sonication and then centrifuged, most of the HrpN_(Ech) protein sedimented with the cell debris. However, soluble HrpN_(Ech) could be released from this material by treatment with 4.5 M guanidine-HCl. This suggested that the protein formed inclusion bodies which could be exploited for purification. As detailed in Example 3, it was found that HrpN_(Ech) reprecipitated when the guanidine-HCl was removed by dialysis against dilute buffer. The HrpN_(Ech) precipitate could be washed and resuspended in buffer, in which it formed a fine suspension. SDS polyacrylamide gel analysis revealed the suspension to be electrophoretically homogeneous HrpN_(Ech) (FIG. 3).

Cell-free lysates from E. coli DH5α(pCPP2172) cells grown in LB medium were infiltrated into tobacco leaves. Necrosis typical of the hypersensitive response developed within 18 hr, whereas leaf panels infiltrated with identically prepared lysates of DH5α(pBluescript SK-) showed no response. The suspension of purified HrpN_(Ech) at a concentration of 336 jig/ml also caused a necrotic response within 18 hrs. that was indistinguishable from that caused by E. chrysanthemi CUCBP5030 or cell-free lysates from E. coli DH5α(pCPP2172) (FIG. 4). Tobacco plants vary in their sensitivity to harpins, and elicitation of the hypersensitive response by HrpN_(Ech) at lower concentrations was found to be variable. Consequently, a concentration of 336 μg/ml was used in all subsequent experiments. The concentration of HrpN_(Ech) that is soluble in apoplastic fluids is unknown. To determine the heat stability of HrpN_(Ech), the suspension of purified protein was incubated at 100° C. for 15 min and then infiltrated into a tobacco leaf. There was no apparent diminution in its ability to elicit the hypersensitive response. These observations indicated that HrpN_(Ech) is sufficient to account for the ability of E. chrysanthemi to elicit the hypersensitive response in tobacco.

Example 8 hrpN_(Ech) Mutants Fail to Elicit the Hypersensitive response in Tobacco

E. coli DH10B(pCPP2142) was mutagenized with Tn5-gusA1 (Sharma, S. B., et al., "Temporal and Spatial Regulation of the Symbiotic Genes of Rhizobium meliloti in Planta Revealed by Transposon TN5-gusA, Genes Develop 4:344-356 (1990), which is incorporated by reference). Plasmid DNA was isolated from kanamycin-resistant colonies and transformed into E. coli DH5S, with selection for kanamycin resistance. Plasmids containing Tn5-gusA1 were analyzed by restriction mapping. Two independent insertions in an 0.82 kb ClaI fragment internal to hrpN_(Ech) were chosen for further study. The precise location and orientation of these insertions was determined by using a primer that hybridizes to Tn5-gusA1 DNA upstream of gusA to sequence into the disrupted E. chrysanthemi DNA (FIG. 1). E. coli DH5α(pCPP2142) cells carrying the Tn5-gusA1 insertion at nucleotide 439 of the hrpN_(Ech) open reading frame (with gusA and hrpN_(Ech) in the same orientation) produced dark blue colonies indicative of β-glucuronidase activity on LM agar supplemented with 5-bromo-4-chloro-3-indolyl β-D-glucuronide. Whether gusA was expressed from an E. chrysanthemi promoter or the vector lac promoter was not determined. The hrpN_(Ech) 439::Tn5-gusA1 and hrpN_(Ech) 546::Tn5-gusA1 mutations were marker-exchanged into the genome of E. chrysanthemi CUCPB5006 (ΔpelABCE) to produce mutants CUCPB5046 and CUCPB5045, respectively. Neither of the hrpN_(Ech) mutants elicited a visible reaction in tobacco leaves (FIG. 5.)

Example 9 E. chrysanthemi hrpN_(Ech) Mutations can be Complemented in trans with hrpN_(Ech) but not with hrpN_(Ea)

The presence of a typical rho-independent terminator just downstream of the hrpN_(Ech) open reading frame suggested that mutations in the gene would not have polar effects on any other genes and that the hypersensitive response elicitation phenotype should be restored by a hrpN_(Ech) subclone. Because pCPP2172 carried 2 kb of E. chrysanthemi DNA in addition to hrpN_(Ech), a precise subclone of the gene was constructed for this purpose. Oligonucleotides were used to PCR-amplify the hrpN_(Ech) open reading frame and to introduce terminal NcoI and XhoI sites. The introduction of the restriction sites resulted in changing the second residue in the protein from glutamine to valine and adding a leucine and a glutamic acid residue to the C-terminus. The resulting DNA fragment was ligated into Xhol/Ncol-digested pSE280, creating pCPP2174, in which hrpN_(Ech) was under control of the vector tac promoter. E. chrysanthemi CUCPB5045(pCPP2174) and CUCPB5046(pCPP2174) possessed hypersensitive response elicitor activity (FIG. 5). Hypersensitive response elicitor activity could also be restored to these mutants by pCPP2142 and pCPP2171, but not by pCPP2141. Thus, the production of HrpN_(Ech) is essential for elicitation of the hypersensitive response by E. chrysanthemi CUCPB5006.

The feasibility of testing the interchangeability of the hrpN genes of E. chrysanthemi and E. amylovora was supported by the observation that hypersensitive response elicitation activity could be restored to hrpN mutants in each species (E. chrysanthemi CUCPB5045 and E. amylovora Ea321T5) by their respective hrpN⁺ subclones (pCPP2142 and pCPP1084). pCPP2142 was used for this purpose, because preliminary immunoblot experiments indicated that the level of hrpN_(Ech) expression by this plasmid, though relatively high, most closely approximated expression of the native hrpN gene in E. amylovora. However, despite good heterologous expression of the hrpN genes, hypersensitive response elicitation activity was not restored in either E. amylovora Ea321T5(pCPP2142) or E. chrysanthemi(pCPP1084). Thus, the genes do not appear to be functionally interchangeable.

Example 10 E. chrysantheml hrpN_(Ech) Mutants have a Reduced Ability to Incite Lesions in Witloof Chicory

The hrpN_(Ech) 439::Tn5-gusA.sub. 1 mutation was marker-exchanged into the genome of wild-type strain AC4150. The resulting mutant, CUCPB5049, was analyzed for its virulence in witloof chicory. Leaves were inoculated at small wounds with 2×10⁴ cells of mutant and wild-type strains, as previously described (Bauer, et al., "Erwinia chrysanthemi hr-p Genes and Their Involvement in Soft Rot Pathogenesis and Elicitation of the Hypersensitive Response," Mol. Plant-Microbe Interact, 7:573-581 (1994), which is incorporated by reference). The level of inoculum corresponded with the experimentally determined ED₅₀ of the wild-type strain for the batch of chicory heads used. The approximate surface area of macerated lesions was determined 72 hr after inoculation. The mutations did not abolish the pathogenicity of E. chrysanthemi, but significantly reduced the number of successful lesions (Table 2).

                  TABLE 2                                                          ______________________________________                                         Effects of hrpN.sub.Ech mutation on the ability of Erwinia                       chrysanthemi to incite lesions on witloof chicory leaves                                         Number of                                                     lesions per 20 Size of lesions                                                Strain inoculations.sup.a (mm.sup.2, mean ± SD).sup.b                     ______________________________________                                         AC4150          16         80 ± 55                                            (wild type)                                                                    CUCPB5049  8.sup.c 89 ± 42                                                  (hrpn.sub.Ech 429::Tn5-gusA1)                                                ______________________________________                                          .sup.a Each witloff chicory leaf was inoculated at two equivalent sites        with 2 × 10.sup.4 bacterial cells: one site received the                 hrpN.sub.Ech mutant, the other the parental wildtype strain; lesions were      indicated by browning and maceration around the site of inoculation.           .sup.b Values represent the product of the length and width of the lesion      .sup.c Different from the wildtype strain (P <0.05), as determined by the      McNemar test (Conover, W. J., "Practial Nonparametric Statistics", 2 ed.,      John Wiley and Sons, New York (1980), which is incorporated by reference)

However, the hrpN_(Ech) mutation had no significant effect on the size o the lesions produced in successful infections.

Example 11 Elicitation of a Rapid Necrosis in Several Plants by E. chrysanthemi is Dependent on HrpN_(Ech)

To determine whether E. chrysanthemi could cause a HrpN_(Ech) -dependent necrosis in plants other than tobacco, a variety of plants were infiltrated with purified HrpN_(Ech) or inoculated with Pel-deficient E. chrysanthemi strains. The strains used were CUCPB5006 and its hrpN_(Ech) 546::Tn5-gusA1 derivative CUCPB5045 and CUCPB5030 (ΔpelABCE outD::TnphoA) and its hrpN_(Ech) 546::Tn5-gusA1 derivative CUCPB5063. The results with Saintpaulia ionantha are shown in FIG. 6 and for all plants are summarized in Table 3.

                                      TABLE 3                                      __________________________________________________________________________     Elicitation of necrosis in various plants by                                     HrpN.sub.Ech and by E. chrysanthemi strains that are deficient                 in Pel production but not HrpN.sub.Ech production                                                             CUCPB5063                                          CUCPB5045  (ΔpelABCE)                                                    (ΔpelABCE CUCPB5030 outD::TnphoA                                        CUCPB5006 hrpN.sub.Ech 546::Tn5- (ΔpelABCE hrpN.sub.Ech 546::                                          Plant HrpN.sub.Ech.sup.a (ΔpelABCE                                      ).sup.b gusA1) outD::TnphoA) Tn5-gusA1)       __________________________________________________________________________     Tobacco                                                                              +    +      -       +      -                                               Tomato + + - + -                                                               Pepper + + - + -                                                               Saintpaulia + + - + -                                                          Petunia + + - + -                                                              Pelargonium + + - + -                                                          Squash - - - - -                                                               Zinnia - - - - -                                                             __________________________________________________________________________      .sup.a Leaves on plants were infiltrated with HrpN.sub.Ech at a                concentration of 336 μg/ml and observed macroscopically 24 hr later fo      necrosis and collapse of the infiltrated area (+) or absence of any            response (-).                                                                  .sup.b Leaves on plants were infiltrated with bacteria at a concentration      of 5 × 10.sup.8 and scored for responses as described above.       

They yield several general observations. Plants responded either to both isolated HrpN_(Ech) and hrpN_(Ech) bacteria or to neither. Plants that responded to either treatment produced a nonmacerated, hypersensitive response-like necrosis that developed between 12 and 24 hr after infiltration. hrpN_(Ech) mutants failed to elicit a response elicited in the plants tested, indicating that residual Pel isozymes or other proteins travelling the Out pathway were not involved in producing the hypersensitive response-like necrosis. The results argue that HrpN_(Ech) is the only elicitor of the hypersensitive response produced by E. chrysanthemi.

E. chrysanthemi was found to produce a protein with many similarities to the harpin of E. amylovora. The two proteins share significant amino acid sequence identity, similar physical properties, and the ability to elicit the hypersensitive response in a variety of plants. Mutations in the hrpN_(Ech) gene indicate that, as with E. amylovora, harpin production is required for elicitation of the hypersensitive response. Furthermore, both harpins contribute to bacterial pathogenicity, albeit to different degrees. HrpN_(Ea) is essential for E. amylovora to produce symptoms in highly susceptible, immature pear fruit (Wei, et al. "Harpin, Elicitor of the Hypersensitive Response Produced by the Plant Pathogen Erwinia amylovora," Science 257:85-88 (1992), which is incorporated by reference), whereas HrpN_(Ech) merely increases the frequency of successful E. chrysanthemi infections in susceptible witloof chicory leaves. Nevertheless, the finding that harpins play some role in the pathogenicity of such disparate pathogens suggests that these proteins have a conserved and widespread function in bacterial plant pathogenesis. We will consider below HrpN_(Ech) with regard to the protein secretion pathways, extracellular virulence proteins, and wide host range of E. chrysanthemi.

E. chrysanthemi secretes proteins by multiple, independent pathways. Several protease isozymes are secreted by the Sec-independent (ABC-transporter or Type I) pathway; pectic enzymes and cellulase are secreted by the Sec-dependent (general secretion or Type II) pathway; and, HrPN_(Ech) is likely to be secreted by the Sec-independent Hrp (Type III) pathway (Salmond, G. P. C., et al., "Secretion of Extracellular Virulence Factors by Plant Pathogenic Bacteria," Annu. Rev. Phytopathol. 32:181-200 (1994), which is incorporated by reference). The expectation that HrpN_(Ech) is secreted by the Hrp pathway is supported by several lines of indirect evidence: (i) Hrp secretion pathway mutants have revealed that other members of this class of glycine-rich, heat-stable elicitor proteins, the E. amylovora HrpN_(Ea), P. syringae pv. syringae HrpZ, and P. solanacearum PopA1 proteins, are secreted by this pathway (He, S. Y., et al., "Pseudomonas syringae pv. syringae Harpin_(Pss) : a Protein that is Secreted via the Hrp Pathway and Elicits the Hypersensitive Response in Plants," (1993); Wei, Z.-M., et al., "Hrpl of Erwinia amylovora Functions in Secretion of Harpin and is a Member of a New Protein Family", J. Bacteriol. 175:7958-7967 (1993); Arlat, M., et al. "PopA1, a Protein Which Induces a Hypersensitive-like Response on Specific Petunia Genotypes, is Secreted via the Hrp Pathway of Pseudomonas solanacarum," EMBO J. 13:543-553 (1994), which are hereby incorporated by reference); (ii) mutation of the E. chrysanthemi homolog of an E. amylovora gene involved in HrpN_(Ea) secretion abolishes the ability of E. chrysanthemi to elicit the hypersensitive response, whereas mutation of the Out (Type II) pathway of E. chrysanthemi does not abolish the hypersensitive response; and (iii) HrpN_(Ech) appears to be the only hypersensitive response elicitor produced by E. chrysanthemi (as discussed further below), suggesting that the effect of the putative hrp secretion gene mutation is on HrpN_(Ech).

Attempts to demonstrate directly hrp-dependent secretion of HrpN_(Ech) have been thwarted by apparent instability of the protein in E. chrysanthemi. Using the cell fractionation and immunoblotting procedures of He, S. Y., et al., "Pseudomonas syringae pv. syringae Harpin_(Pss) : a Protein that is Secreted via the Hrp Pathway and Elicits the Hypersensitive Response in Plants," Cell 73:1255-66 (1993), which is hereby incorporated by reference, and polyclonal anti-HrpN_(Ea) antibodies that cross-react with HrpN_(Ech) (Wei, Z.-M, et al., "Harpin, Elicitor of the Hypersensitive Response Produced by the Plant Pathogen Erwinia amylovora," Science 257:85-88 (1992), which is incorporated by reference), we have observed the presence of HrpN_(Ech) in the cell-bound fraction of E. chrysanthemi. However, some culture preparations unexpectedly lack the protein, and no preparations reveal accumulation of the protein in the culture supernatant fraction. It is possible that HrpN_(Ech) aggregates upon secretion and, therefore, precipitates from the medium. It is interesting that several of the Yersinia spp. Yop virulence proteins aggregate in the medium upon secretion via the Type III pathway (Michiels, T., et al., "Secretion of Yop Proteins by Yersiniae," Infect. Immun. 58:2840-2849 (1990), which is incorporated by reference). Similarly, HrpN_(Ea) has a propensity to form aggregates or to associate with an insoluble membrane fraction (Wei, Z.-M, et al., "Harpin, Elicitor of the Hypersensitive Response Produced by the Plant Pathogen Erwinia amylovora," Science 257:85-88 (1992), which is incorporated by reference).

It is significant that there is little difference in the plant interaction phenotypes of E. chrysanthemi mutants deficient in either HrpN_(Ech) or a putative component of the Hrp secretion pathway (Bauer, D. W., et al., "Erwinia chrysanthemi hrp Genes and Their Involvement in Soft Rot Pathogenesis and Elicitation of the Hypersensitive Response," Mol. Plant-Microbe Interact, 7:573-581 (1994), which is incorporated by reference). Both mutations abolish the ability of Pel-deficient strains to elicit the hypersensitive response, and they both reduce the frequency of successful infections incited by fully pectolytic strains in witloof chicory leaves without affecting the size of the macerated lesions that do develop. This pattern contrasts with that observed with mutations affecting Pel isozymes and the Out pathway. Maceration virulence is merely reduced by individual pel mutations, whereas it is abolished by out mutations. This is because multiple Pel isozymes (and possibly other enzymes) contribute quantitatively to virulence, but all of the Pel isozymes appear to be dependent on the Out pathway for secretion from the bacterial cell. The simplest interpretation of the observations with E. chrysanthemi hrp mutants is that HrpN_(Ech) is the only protein travelling the Hrp pathway that has a detectable effect on the interaction of E. chrysanthemi EC16 with the plants tested.

The primacy of HrpN_(Ech) in the E. chrysanthemi Hrp system is further supported by the observations that hrpN_(Ech) mutants failed to elicit necrosis in any of the several plants tested and that all plants responding with apparent hypersensitivity to HrpN_(Ech) ⁺ strains also responded to isolated HrpN_(Ech). Several of the plants sensitive to HrpN_(Ech) are also susceptible to bacterial soft rots. This is particularly significant for Saintpaulia, whose interactions with E. chrysanthemi have been extensively studied (Barras, F., et al., "Extracellular Enzymes and Pathogenesis of Soft-rot Erwinia," Annu. Rev. Phytopathol. 32:201-234 (1994), which is hereby incorporated by reference). Thus, HrpN_(Ech) elicits hypersensitive response-like responses in plants that are susceptible to E. chrysanthemi infections under appropriate environmental conditions. The significance of this for the wide host range of the bacterium requires further investigation, and virulence tests with hrpN_(Ech) mutants and additional susceptible plants are needed to determine the general importance of HrpN_(Ech) and the Hrp system in E. chrysanthemi. For example, the present data do not address the possibility that other proteins secreted by the Hrp pathway, which are not elicitors of the hypersensitive response in the plants tested, may contribute to pathogenesis in hosts other than witloof chicory.

An important question is whether bacteria expressing heterologous harpins will be altered in pathogenicity. The hrpN genes of E. chrysanthemi and E. amylovora are particularly attractive for experiments addressing this because of the similarity of the harpins and the dissimilarity of the diseases produced by these bacteria. Unfortunately, attempts to restore the hypersensitive response phenotype to E. chrysanthemi and E. amylovora hrpN mutants with heterologous hrpbr subclones failed. Since the hrpN genes in each subclone successfully complemented hrpN mutations in homologous bacteria and were expressed in heterologous bacteria, the problem is most likely the secretion of the harpins by heterologous Hrp systems. A similar problem has been encountered with heterologous secretion of Pel and cellulase via the Out pathway in E. chxysanthemi and E. carotovora, species that are more closely related to each. other in this rather heterogenous genus than are E. chrysanthemi and E. amylovora (He, S. Y., et al., "Cloned Erwinia chrysanthemi out Genes Enable Escherichia coli to Selectively Secrete a Diverse Family of Heterologous Proteins to its Milieu," Proc. Natl. Acad. Sci. U.S.A. 88:1079-1083 (1991); Py, B., et al., "Secretion of Cellulases in Erwinia chrysanthemi and E. carotovora is Species-specific," FEMS Microbiol. Lett. 79:315-322 (1991), which are hereby incorporated by reference).

In conclusion, two classes of proteins contribute to the pathogenicity of E. chrysanthemi--a single harpin and a battery of plant cell wall-degrading pectic enzymes. The observation that such a highly pectolytic organism also produces a harpin suggests the fundamental importance of harpins in the pathogenicity of gram-negative bacteria. The observation that a hrpN_(Ech) ::Tn5-gusA1 mutation reduced the ability of a fully pectolytic strain of E. chrysanthemi to initiate lesions in susceptible chicory leaves, but did not reduce the size of lesions that did develop, suggests that HrpN_(Ech) contributes specifically to an early stage of pathogenesis. An attractive possibility is that HrpN_(Ech) releases nutrients to the apoplast for bacterial nutrition before the pel genes are fully expressed (Collmer, A., et al., "Erwinia chrysanthemi and pseudomonas syringae: Plant Pathogens Trafficking in Virulence Proteins," pages 43-78 in: Current Topics in Microbiology and Immunology, Vol. 192: Bacterial Pathogenesis of Plants and Animals--Molecular and Cellular Mechanisms (1994), which is incorporated by reference). Patterns of pel and hrpN_(Ech) expression in plants will likely yield further clues to the role of the E. chrysanthemi harpin in soft rot pathogenesis.

Although the invention has been described in detail for the purpose of illustration, it is understood that such detail is solely for that purpose, and variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention which is defined by the following claims.

    __________________________________________________________________________     #             SEQUENCE LISTING                                                    - -  - - (1) GENERAL INFORMATION:                                              - -    (iii) NUMBER OF SEQUENCES: 6                                            - -  - - (2) INFORMATION FOR SEQ ID NO:1:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 2141 base - #pairs                                                 (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: DNA (genomic)                                      - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                - - CGATTTTACC CGGGTGAACG TGCTATGACC GACAGCATCA CGGTATTCGA CA -             #CCGTTACG     60                                                                  - - GCGTTTATGG CCGCGATGAA CCGGCATCAG GCGGCGCGCT GGTCGCCGCA AT -             #CCGGCGTC    120                                                                  - - GATCTGGTAT TTCAGTTTGG GGACACCGGG CGTGAACTCA TGATGCAGAT TC -             #AGCCGGGG    180                                                                  - - CAGCAATATC CCGGCATGTT GCGCACGCTG CTCGCTCGTC GTTATCAGCA GG -             #CGGCAGAG    240                                                                  - - TGCGATGGCT GCCATCTGTG CCTGAACGGC AGCGATGTAT TGATCCTCTG GT -             #GGCCGCTG    300                                                                  - - CCGTCGGATC CCGGCAGTTA TCCGCAGGTG ATCGAACGTT TGTTTGAACT GG -             #CGGGAATG    360                                                                  - - ACGTTGCCGT CGCTATCCAT AGCACCGACG GCGCGTCCGC AGACAGGGAA CG -             #GACGCGCC    420                                                                  - - CGATCATTAA GATAAAGGCG GCTTTTTTTA TTGCAAAACG GTAACGGTGA GG -             #AACCGTTT    480                                                                  - - CACCGTCGGC GTCACTCAGT AACAAGTATC CATCATGATG CCTACATCGG GA -             #TCGGCGTG    540                                                                  - - GGCATCCGTT GCAGATACTT TTGCGAACAC CTGACATGAA TGAGGAAACG AA -             #ATTATGCA    600                                                                  - - AATTACGATC AAAGCGCACA TCGGCGGTGA TTTGGGCGTC TCCGGTCTGG GG -             #CTGGGTGC    660                                                                  - - TCAGGGACTG AAAGGACTGA ATTCCGCGGC TTCATCGCTG GGTTCCAGCG TG -             #GATAAACT    720                                                                  - - GAGCAGCACC ATCGATAAGT TGACCTCCGC GCTGACTTCG ATGATGTTTG GC -             #GGCGCGCT    780                                                                  - - GGCGCAGGGG CTGGGCGCCA GCTCGAAGGG GCTGGGGATG AGCAATCAAC TG -             #GGCCAGTC    840                                                                  - - TTTCGGCAAT GGCGCGCAGG GTGCGAGCAA CCTGCTATCC GTACCGAAAT CC -             #GGCGGCGA    900                                                                  - - TGCGTTGTCA AAAATGTTTG ATAAAGCGCT GGACGATCTG CTGGGTCATG AC -             #ACCGTGAC    960                                                                  - - CAAGCTGACT AACCAGAGCA ACCAACTGGC TAATTCAATG CTGAACGCCA GC -             #CAGATGAC   1020                                                                  - - CCAGGGTAAT ATGAATGCGT TCGGCAGCGG TGTGAACAAC GCACTGTCGT CC -             #ATTCTCGG   1080                                                                  - - CAACGGTCTC GGCCAGTCGA TGAGTGGCTT CTCTCAGCCT TCTCTGGGGG CA -             #GGCGGCTT   1140                                                                  - - GCAGGGCCTG AGCGGCGCGG GTGCATTCAA CCAGTTGGGT AATGCCATCG GC -             #ATGGGCGT   1200                                                                  - - GGGGCAGAAT GCTGCGCTGA GTGCGTTGAG TAACGTCAGC ACCCACGTAG AC -             #GGTAACAA   1260                                                                  - - CCGCCACTTT GTAGATAAAG AAGATCGCGG CATGGCGAAA GAGATCGGCC AG -             #TTTATGGA   1320                                                                  - - TCAGTATCCG GAAATATTCG GTAAACCGGA ATACCAGAAA GATGGCTGGA GT -             #TCGCCGAA   1380                                                                  - - GACGGACGAC AAATCCTGGG CTAAAGCGCT GAGTAAACCG GATGATGACG GT -             #ATGACCGG   1440                                                                  - - CGCCAGCATG GACAAATTCC GTCAGGCGAT GGGTATGATC AAAAGCGCGG TG -             #GCGGGTGA   1500                                                                  - - TACCGGCAAT ACCAACCTGA ACCTGCGTGG CGCGGGCGGT GCATCGCTGG GT -             #ATCGATGC   1560                                                                  - - GGCTGTCGTC GGCGATAAAA TAGCCAACAT GTCGCTGGGT AAGCTGGCCA AC -             #GCCTGATA   1620                                                                  - - ATCTGTGCTG GCCTGATAAA GCGGAAACGA AAAAAGAGAC GGGGAAGCCT GT -             #CTCTTTTC   1680                                                                  - - TTATTATGCG GTTTATGCGG TTACCTGGAC CGGTTAATCA TCGTCATCGA TC -             #TGGTACAA   1740                                                                  - - ACGCACATTT TCCCGTTCAT TCGCGTCGTT ACGCGCCACA ATCGCGATGG CA -             #TCTTCCTC   1800                                                                  - - GTCGCTCAGA TTGCGCGGCT GATGGGGAAC GCCGGGTGGA ATATAGAGAA AC -             #TCGCCGGC   1860                                                                  - - CAGATGGAGA CACGTCTGCG ATAAATCTGT GCCGTAACGT GTTTCTATCC GC -             #CCCTTTAG   1920                                                                  - - CAGATAGATT GCGGTTTCGT AATCAACATG GTAATGCGGT TCCGCCTGTG CG -             #CCGGCCGG   1980                                                                  - - GATCACCACA ATATTCATAG AAAGCTGTCT TGCACCTACC GTATCGCGGG AG -             #ATACCGAC   2040                                                                  - - AAAATAGGGC AGTTTTTGCG TGGTATCCGT GGGGTGTTCC GGCCTGACAA TC -             #TTGAGTTG   2100                                                                  - - GTTCGTCATC ATCTTTCTCC ATCTGGGCGA CCTGATCGGT T    - #                       - # 2141                                                                      - -  - - (2) INFORMATION FOR SEQ ID NO:2:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 338 amino - #acids                                                 (B) TYPE: amino acid                                                           (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: protein                                            - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                - - Met Gln Ile Thr Ile Lys Ala His Ile Gly Gl - #y Asp Leu Gly Val Ser       1               5   - #                10  - #                15                - - Gly Leu Gly Ala Gln Gly Leu Lys Gly Leu As - #n Ser Ala Ala Ser Ser                   20      - #            25      - #            30                    - - Leu Gly Ser Ser Val Asp Lys Leu Ser Ser Th - #r Ile Asp Lys Leu Thr               35          - #        40          - #        45                        - - Ser Ala Leu Thr Ser Met Met Phe Gly Gly Al - #a Leu Ala Gln Gly Leu           50              - #    55              - #    60                            - - Gly Ala Ser Ser Lys Gly Leu Gly Met Ser As - #n Gln Leu Gly Gln Ser       65                  - #70                  - #75                  - #80         - - Phe Gly Asn Gly Ala Gln Gly Ala Ser Asn Le - #u Leu Ser Val Pro Lys                       85  - #                90  - #                95                - - Ser Gly Gly Asp Ala Leu Ser Lys Met Phe As - #p Lys Ala Leu Asp Asp                   100      - #           105      - #           110                   - - Leu Leu Gly His Asp Thr Val Thr Lys Leu Th - #r Asn Gln Ser Asn Gln               115          - #       120          - #       125                       - - Leu Ala Asn Ser Met Leu Asn Ala Ser Gln Me - #t Thr Gln Gly Asn Met           130              - #   135              - #   140                           - - Asn Ala Phe Gly Ser Gly Val Asn Asn Ala Le - #u Ser Ser Ile Leu Gly       145                 1 - #50                 1 - #55                 1 -       #60                                                                               - - Asn Gly Leu Gly Gln Ser Met Ser Gly Phe Se - #r Gln Pro Ser Leu         Gly                                                                                              165  - #               170  - #               175              - - Ala Gly Gly Leu Gln Gly Leu Ser Gly Ala Gl - #y Ala Phe Asn Gln Leu                   180      - #           185      - #           190                   - - Gly Asn Ala Ile Gly Met Gly Val Gly Gln As - #n Ala Ala Leu Ser Ala               195          - #       200          - #       205                       - - Leu Ser Asn Val Ser Thr His Val Asp Gly As - #n Asn Arg His Phe Val           210              - #   215              - #   220                           - - Asp Lys Glu Asp Arg Gly Met Ala Lys Glu Il - #e Gly Gln Phe Met Asp       225                 2 - #30                 2 - #35                 2 -       #40                                                                               - - Gln Tyr Pro Glu Ile Phe Gly Lys Pro Glu Ty - #r Gln Lys Asp Gly         Trp                                                                                              245  - #               250  - #               255              - - Ser Ser Pro Lys Thr Asp Asp Lys Ser Trp Al - #a Lys Ala Leu Ser Lys                   260      - #           265      - #           270                   - - Pro Asp Asp Asp Gly Met Thr Gly Ala Ser Me - #t Asp Lys Phe Arg Gln               275          - #       280          - #       285                       - - Ala Met Gly Met Ile Lys Ser Ala Val Ala Gl - #y Asp Thr Gly Asn Thr           290              - #   295              - #   300                           - - Asn Leu Asn Leu Arg Gly Ala Gly Gly Ala Se - #r Leu Gly Ile Asp Ala       305                 3 - #10                 3 - #15                 3 -       #20                                                                               - - Ala Val Val Gly Asp Lys Ile Ala Asn Met Se - #r Leu Gly Lys Leu         Ala                                                                                              325  - #               330  - #               335              - - Asn Ala                                                                    - -  - - (2) INFORMATION FOR SEQ ID NO:3:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 21 base - #pairs                                                   (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: cDNA                                               - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                - - TGACCTGCAG CCAAGCTTTC C           - #                  - #                       - #21                                                                    - -  - - (2) INFORMATION FOR SEQ ID NO:4:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 30 base - #pairs                                                   (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: cDNA                                               - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:                                - - AGTACCATGG TTATTACGAT CAAAGCGCAC         - #                  - #                30                                                                       - -  - - (2) INFORMATION FOR SEQ ID NO:5:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 28 base - #pairs                                                   (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: cDNA                                               - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:                                - - AGATCTCGAG GGCGTTGGCC AGCTTACC         - #                  - #                  28                                                                       - -  - - (2) INFORMATION FOR SEQ ID NO:6:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 1023 base - #pairs                                                 (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: DNA (genomic)                                      - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:                                - - ATGCAAATTA CGATCAAAGC GCACATCGGC GGTGATTTGG GCGTCTCCGG TC -              #TGGGGCTG     60                                                                  - - GGTGCTCAGG GACTGAAAGG ACTGAATTCC GCGGCTTCAT CGCTGGGTTC CA -             #GCGTGGAT    120                                                                  - - AAACTGAGCA GCACCATCGA TAAGTTGACC TCCGCGCTGA CTTCGATGAT GT -             #TTGGCGGC    180                                                                  - - GCGCTGGCGC AGGGGCTGGG CGCCAGCTCG AAGGGGCTGG GGATGAGCAA TC -             #AACTGGGC    240                                                                  - - CAGTCTTTCG GCAATGGCGC GCAGGGTGCG AGCAACCTGC TATCCGTACC GA -             #AATCCGGC    300                                                                  - - GGCGATGCGT TGTCAAAAAT GTTTGATAAA GCGCTGGACG ATCTGCTGGG TC -             #ATGACACC    360                                                                  - - GTGACCAAGC TGACTAACCA GAGCAACCAA CTGGCTAATT CAATGCTGAA CG -             #CCAGCCAG    420                                                                  - - ATGACCCAGG GTAATATGAA TGCGTTCGGC AGCGGTGTGA ACAACGCACT GT -             #CGTCCATT    480                                                                  - - CTCGGCAACG GTCTCGGCCA GTCGATGAGT GGCTTCTCTC AGCCTTCTCT GG -             #GGGCAGGC    540                                                                  - - GGCTTGCAGG GCCTGAGCGG CGCGGGTGCA TTCAACCAGT TGGGTAATGC CA -             #TCGGCATG    600                                                                  - - GGCGTGGGGC AGAATGCTGC GCTGAGTGCG TTGAGTAACG TCAGCACCCA CG -             #TAGACGGT    660                                                                  - - AACAACCGCC ACTTTGTAGA TAAAGAAGAT CGCGGCATGG CGAAAGAGAT CG -             #GCCAGTTT    720                                                                  - - ATGGATCAGT ATCCGGAAAT ATTCGGTAAA CCGGAATACC AGAAAGATGG CT -             #GGAGTTCG    780                                                                  - - CCGAAGACGG ACGACAAATC CTGGGCTAAA GCGCTGAGTA AACCGGATGA TG -             #ACGGTATG    840                                                                  - - ACCGGCGCCA GCATGGACAA ATTCCGTCAG GCGATGGGTA TGATCAAAAG CG -             #CGGTGGCG    900                                                                  - - GGTGATACCG GCAATACCAA CCTGAACCTG CGTGGCGCGG GCGGTGCATC GC -             #TGGGTATC    960                                                                  - - GATGCGGCTG TCGTCGGCGA TAAAATAGCC AACATGTCGC TGGGTAAGCT GG -             #CCAACGCC   1020                                                                  - - TGA                  - #                  - #                  - #                1023                                                                 __________________________________________________________________________ 

What is claimed:
 1. An isolated protein or polypeptide from Erwinia chrysanthemi which elicits a hypersensitive response in plants, wherein the isolated protein or polypeptide has an amino acid sequence of SEQ. ID. No.
 2. 2. An isolated protein or polypeptide according to claim 1, wherein the protein or polypeptide has a molecular weight of 32 to 36 kDa.
 3. An isolated protein or polypeptide according to claim 1, wherein the protein or polypeptide is heat stable, has a glycine content of greater than 16%, and contains no cysteine.
 4. An isolated protein or polypeptide according to claim 1, wherein the protein or polypeptide is purified.
 5. An isolated protein or polypeptide according to claim 1, wherein the protein or polypeptide is recombinant. 